Purity: Timing of Prayers with Jabirah Wudu or Ghusl

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 340:

If someone must perform prayers with jabīrah wuḍūʾ or ghusl, in the event that he knows his legitimate excuse for not performing normal wuḍūʾ or ghusl will remain valid until the end of the time for prayers, he can perform prayers at the beginning of their time. However, if he has hope that his legitimate excuse will expire before the end of the time for prayers, it is better that he wait; and in the event that his legitimate excuse remains valid, he must perform prayers with jabīrah wuḍūʾ or ghusl at the end of the time for prayers. If he performed prayers at the beginning of their time and his legitimate excuse expired by the end of the time for prayers, the recommended precaution is that he should perform wuḍūʾ or ghusl and repeat the prayers.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)